The AE anion exchanger gene family encodes complex polytopic transmembrane polypeptides that contribute to regulation of intracellular pH (pHi), cell [Cl-], and cell volume through their mediation of electroneutral Cl-/HCO3- exchange. AE-mediated Cl-/HCO3- exchange in polarized epithelia also regulates secretion and reabsorption of proton equivalents and of Cl-. AE-mediated Cl-/HCO3-exchange is thought to be of widespread physiological importance in many cell types. AE1 deficiencies have been particularly associated with hereditary syndromes of spherocytic anemia and of distal renal tubular acidosis. Deficiencies of AE2 or AE3 activity have yet to be defined. Deficiency of a different Cl-/HCO3- exchange activity leads to congenital chloride diarrhea. This competitive continuation grant application proposes to extend past and current experiments by pursuit of the following Specific Aims: 1. Further define structural loci of the regulatory differences among AE isoforms, especially AE1 and AE2. 2. Study natural variants of the AE genes and a different class of anion exchanger for clues about ion translocation pathways and mechanisms. These will include: a. AE1 mutations that cosegregate and likely contribute to heritable distal renal tubular acidosis b. AE polypeptides of genetically related fish that live in river or in soda lake environments c. a more distantly related member of the bicarbonate-transporter superfamily cloned from yeast d. the unrelated sulfate transporter DRA that when mutated results in congenital chloride diarrhea. 3. Further compare and define the mechanisms of electroneutral and electrogenic anion exchange mediated by AE1 E699Q and likely mediated by AE2 E1007Q. 4. Apply directed mutagenesis to define the residues of AE1 and AE2 that contribute to binding and transport of substrate anions and to deduce constraints on secondary and tertiary structure of AE polypeptides. 5. Define aspects of transcriptional and translational regulation of AE gene products in kidney of mutant and parental mouse strains and in cultured kidney cells.